As well known to those skilled in the art, the signal feed structure of conventional small-sized antennas for radio communication equipment has been designed in that a signal is directly fed to a coaxial line. Such a signal feed structure includes two types: a monopole type wherein a signal is fed to the plus portion of a coaxial line, and a dipole type wherein a signal is fed to both the plus and minus portions of a coaxial line.
However, the above-mentioned signal feed structure for antennas is problematic in that it results in an unbalance between signal feed lines of an antenna, thus practically making it difficult to match the impedance of the antenna. Such a signal feed structure also causes the contact portions between the antenna and the signal feed lines to be frequently changed, thus allowing the characteristics of the antenna to be undesirably changed. This results in a reduction in the antenna efficiency.
FIG. 4 shows the construction of a conventional wide-band helical antenna disclosed in U.S. Pat. No. 4,772,895. The above wide-band helical antenna is designed to broaden frequency response and comprises a feed port including a signal feed portion and a ground portion. The above antenna also comprises two helically configured conductive elements: first and second elements 200 and 400. The first element 200 has opposite ends, and exhibits a first pitch and a first electrical length. One end of the first element 200 is coupled to the signal feed portion of the feed port. On the other hand, the second element 400 has opposite ends, and exhibits a second pitch and a second electrical length. The second element 400 is coaxially wound around a portion of the first element. One end of the second element 400 is coupled to the ground portion of the feed port. The second pitch is equal to approximately one half of the first pitch, while the second electrical length is equal to approximately one third of the first electrical length. The above antenna further comprises a cylindrical spacer means 300. The above spacer means 300 is coaxially situated between the first and second elements 200 and 400, thus electrically insulating the two elements 200 and 400. The spacer means 300 is also sufficiently thin such that the first element is tightly coupled to the second element so as to broaden the frequency response exhibited by the first element.
In the above wide-band helical antenna, the spacer means, coaxially situated between the first and second helical elements positioned inside and outside of the antenna respectively, is used as a contact means for allowing the two elements to be coupled together. However, the above antenna is not designed to overcome the unbalance between the signal feed lines experienced in conventional antennas, thereby reducing the antenna efficiency. Another problem associated with the above wide-band helical antenna resides in that it is almost impossible to make a small-sized antenna.